1. Field of the Invention
The present invention relates to an improved lithographic mask pattern, a projection exposure system for the mask pattern, and the manufacture of the mask pattern.
2. Related Background Art
A lithographic technique is generally used in formation of a circuit pattern such as a semiconductor element. This process employs a method of transferring a reticle (mask) pattern onto a sample substrate such as a semiconductor wafer. A photoresist having photosensitive properties is applied to the surface of the sample substrate, and a circuit pattern is transferred to the photoresist in accordance with an illumination light image, i.e., the shape of a transparent pattern of the reticle pattern. In a projection exposure apparatus (e.g., a stepper), an image of the circuit pattern depicted on the reticle, which is to be transferred, is focused on the sample substrate (wafer) through a projection optical system.
FIG. 1 shows a schematic arrangement of a conventional general projection exposure apparatus (stepper). In this conventional projection exposure apparatus, light passing through a Fourier transform plane 15 for reticle patterns 12 has an almost uniform intensity distribution within a circular area (or a rectangular area) centered on the optical axis of an illumination optical system. Illumination light L.sub.0 is limited to illumination light L.sub.10 having a predetermined shape by an aperture stop (spatial filter) 15b in the illumination optical system. The illumination light L.sub.10 illuminates the patterns 12 of a reticle R through a condenser lens CL.
In this case, the spatial filter 15b is located at or near a Fourier transform plane 15 (to be referred to as an illumination system pupil plane 15 hereinafter) for the reticle patterns 12. The spatial filter 15b causes an opening having an almost circular shape centered on an optical axis AX of a projection optical system to limit a secondary source (surface light source) image formed within the pupil plane to a circular image. The illumination light passing through the reticle patterns 12 can focus the reticle patterns 12 on a resist layer on a wafer W through a projection optical system PL. Solid lines representing rays represent a 0th-order light component Do emerging from one point of the reticle pattern 12. At this time, a ratio of the numerical aperture of the illumination optical system (15b and CL) to the reticle-side numerical aperture of the projection optical system PL, i.e., a .sigma. value, is determined by the aperture stop (e.g., the aperture size of the spatial filter 15b). The .sigma. value generally falls within the range of about 0.5 to about 0.6.
A conventional mask pattern is a pattern having the same shape as or similar shape to a pattern to be formed on a wafer as, e.g., a semiconductor integrated circuit pattern. A mask used for one-to-one exposure (e.g., a contact scheme, a proximity scheme, or a mirror projection scheme) has a pattern having the same shape (congruence) as a pattern to be formed on the wafer. A mask used for reduction projection exposure (e.g., a stepper scheme) has a larger pattern than a pattern to be formed on the wafer in accordance with a reduction ratio. If a reduction ratio is 1/5, the size of the pattern on the reticle is five times that of the pattern on the wafer (mask-side conversion).
Since the .sigma. value (coherence factor) of the illumination system in the conventional projection exposure apparatus falls within the range of 0.5 to 0.6, as described above, coherency on the reticle is low. Interference between light components between adjacent patterns does not pose any problem.
The illumination light L.sub.10 is diffracted by the patterns 12 formed on the reticle R into the 0th-order diffracted light component Do, a (+) 1st-order diffracted light component Dp, and a (-) 1st-order diffracted light component Dm from the patterns 12. These diffracted light components (Do, Dm, and Dp) are focused by the projection optical system PL to form interference fringes on the wafer W. These interference fringes represent images of the patterns 12. At this time, an angle .theta. (reticle side) formed between the 0th-order diffracted light component Do and the (.+-.) 1st-order diffracted light components Dp and Dm is determined by sin.theta.=.lambda./P (where .lambda. is the exposure wavelength and P is the pattern pitch).
When a pattern pitch decreases, the sin.theta. increases. When the sin.theta. becomes larger than the reticle-side numerical aperture (NA.sub.R) of the projection optical system PL, the (.+-.) 1st-order diffracted light components Dp and Dm cannot pass through the projection optical system PL. At this time, only the 0th-order diffracted light component Do reaches the wafer W, and interference fringes are not formed thereon. That is, if condition sin.theta.&gt;NA.sub.R is established, the images of the patterns 12 cannot be obtained. As a result, the patterns 12 cannot be transferred onto the wafer W.
Judging from the above description, in a normal projection exposure apparatus, the pitch P satisfying sin.theta.=.lambda./p.perspectiveto.NA.sub.R is given as follows: EQU P.perspectiveto..lambda./NA.sub.R ( 1)
Since the minimum pattern size is 1/2 the pitch P, the minimum pattern size becomes about 0.5.multidot..lambda./NA.sub.R. In the practical photolithographic process, however, a given depth of focus is required due to wafer warping, an influence of a step formed in the wafer in the process, and the thickness of the photoresist itself. For this reason, a practical minimum resolution pattern size is represented by k.multidot..lambda./NA where k is the process coefficient which falls within the range of about 0.6 to about 0.8. Since a ratio of the reticle-side numerical aperture NA.sub.R to the wafer-side numerical aperture NA.sub.W is almost equal to the focusing magnification of the projection optical system, the minimum resolution pattern size becomes k.multidot..lambda./NA.sub.R and the minimum resolution pattern size on the wafer becomes k.multidot..lambda./NA.sub.W =k.multidot..lambda./B.multidot.NA.sub.R (where B is the focusing magnification (reduction factor)).
In order to transfer a finer pattern, an exposure light source having a shorter wavelength or a projection optical system having a larger numerical aperture must be used. It is possible to optimize both the exposure wavelength and the numerical aperture. Alternatively, a so-called phase shifting reticle is proposed in Japanese Patent Publication No. 62-50811, in which, of all transmission parts of the circuit pattern of the reticle, the phase of light transmitted from a specific part is shifted from that from an adjacent part by .pi.. When this phase shift reticle is used, a finer pattern can be transferred.
The same technique as in Japanese Patent Publication No. 62-50811 is also described in Japanese Patent Publication No. 62-59296 (priority claimed to U.S. Ser. No. 365,672 on Apr. 5, 1982).
In the conventional projection exposure apparatus, however, when the illumination light has a shorter wavelength than that currently used, a high-power laser source for ultraviolet rays is required, as proposed in U.S. Pat. No. 4,820,899 or 4,884,101. In addition, "when the wavelength is 200 nm or less,"; the wavelength is 200 nm or less, the existing projection optical system including a diffractive element cannot be used at the wavelength of 200 nm or less because an appropriate material which can be used as a transmission optical member does not exist.
The numerical aperture of the state-of-the-art projection optical system is almost a theoretical limit, and a significant increase does not seem a realistic possibility; the numerical aperture is assumed not to. Even if the numerical aperture can be larger than that currently used, the depth of focus abruptly decreases with an increase in numerical aperture (N.A.), "and the depth of focus used in"; and the depth of focus used in, resulting in inconvenience.
On the other hand, the phase shift reticle is expensive due to the complicated manufacturing process, and neither a test method nor a correction method is established, thus posing many problems. In addition, when a phase shift reticle is used, the .sigma. value as an illumination condition must be small, as disclosed in Japanese Patent Publication No. 62-59296.
A projection transfer technique having a higher resolving power than that in the conventional technique by using an existing projection exposure apparatus but without using a phase shift reticle is proposed in U.S. Pat. No. 4,947,413. In U.S. Pat. No. 4,947,413, a spatial filter is arranged on a Fourier transform plane within a projection optical system, and two specific diffracted light components (e.g., the (+) 1st- and (-) 1st-order diffracted light components, or the 0th-order diffracted light component and one of the (.+-.) 1st-order diffracted light components) generated from the reticle patterns pass toward the wafer.
The present inventor filed U.S. Ser. No. 791,138 (Nov. 13, 1991) describing an improved practical projection transfer technique as compared that described in U.S. Pat. No. 4,947,413. According to this disclosed technique, a high resolving power and a large depth of focus can be obtained without arranging a spatial filter on the Fourier transform plane of the projection optical system. Although the principle of this technique will be described in detail later, the concept essentially different from that in U.S. Pat. No. 4,947,413 is to concentrate the intensity of exposure illumination light at four positions of the Fourier transform plane in the illumination optical system, so that a two-dimensional periodic pattern on a reticle can be projected at a high resolving power with a high contrast. A method of determining the above four positions is a characteristic feature, which cannot be anticipated from U.S. Pat. No. 4,947,413.
In the present invention, a technique for obtaining better image quality than that in a conventional normal focusing scheme by improving only the illumination optical system is called SHRINC (Super High Resolution by IllumiNation Control). Another form of SHRINC incorporates formation in which the shape of a secondary source formed on the Fourier transform plane of the illumination optical system is set as an annular shape.
As described above, although the resolving power and the depth of focus certainly increase in accordance with the SHRINC method and the phase shift method, this applies to only periodic patterns. Satisfactory results are not necessarily obtained for portions having no periodicity, and the causes of problems of the respective techniques will be briefly summarized as follows.
(A) Phase Shift Method
Since the .sigma. value of the illumination optical system of the conventional projection exposure apparatus shown in FIG. 1 is relatively as large as 0.5 to 0.7, coherency of the illumination light on the reticle pattern surface is low. For this reason, adjacent patterns do not substantially influence each other regardless of the type of pattern adjacent to a specific pattern.
In order to maximally enhance the effect of the phase shift method, the .sigma. value of the illumination optical system must be set as small as about 0.2 to about 0.3. For this reason, coherency of the illumination light on the reticle increases, and the width of a resist line upon exposure and transfer of one pattern is influenced by an adjacent pattern.
According to an experimental conclusion, in the phase shift method, the resist line width of a line pattern located at an end position of a periodic pattern in the periodic direction, or an isolated pattern tends to be smaller than that of the periodic pattern. The above conclusion is based on an assumption that patterns to be compared with each other have the same size on the reticle and are exposed and transferred with the same exposure amount, as a matter of course.
In the phase shift method, in order to obtain desired line widths of both the periodic pattern portion and the isolated pattern portion as resist images (i.e., the circuit pattern sizes obtained upon pattern etching), the shapes of the respective patterns of the reticle patterns must be corrected beforehand.
A technique for adding auxiliary patterns near the vertices of a small square so as to clearly expose a four corners of, e.g., a small square transmission pattern (contact hole pattern) has been reported. However, this technique does not propose the correction in consideration of interaction between necessary patterns. In order to enhance the effect of the phase shift method, a method of forming an auxiliary pattern near an original pattern (Papers of the Institute of Applied Physics, 1988 Autumn Meeting). However correction is not made in consideration of the interaction between necessary patterns in this method either. The correction methods described above are based on empirical manual operations and are not automatic correction methods in which algorithms are established.
(B) SHRINC Method
The SHRINC method disclosed in U.S. Ser. No. 791,138 filed by the present inventor is illustrated in FIG. 2. FIG. 2 additionally illustrates the SHRINC method for providing an annular illumination light distribution.
Referring to FIG. 2, a projection optical system PL is located between a reticle R and a photosensitive substrate (wafer) W, and a pattern on the reticle R is focused on the wafer W. At this time, the reticle R is illuminated with exposure illumination light through a condenser lens CL in an illumination optical system. A spatial filter SF.sub.1 for shaping illumination light IL into annular light or a spatial filter SF.sub.2 having small openings at two to four discrete positions is arranged within the Fourier transform plane (the same plane as the illumination system pupil plane 15 in FIG. 1) of the illumination optical system. Light components parallel to the optical axis of the projection lens or projection optical system PL are eliminated by the spatial filter SF.sub.1 or SF.sub.2, and illumination light having light components having a specific angle reaches the reticle R. The spatial filter SF.sub.1 or SF.sub.2 is located on the Fourier transform plane of the illumination optical system, as described above, and at the same time is conjugate with a pupil plane ep of the projection optical system PL.
When the inclination characteristics of the illumination light with respect to the reticle R are converted into specific ones by the illumination optical system, the resolving power and the depth of focus of the pattern can be improved by about 10% to 40%.
When the exposure apparatus which employs the SHRINC method described above is used, the incident direction of the illumination light on the reticle is limited to the direction different from that in the conventional case. For this reason, coherency different from that in the conventional case occurs in the illumination light on the reticle, and the interaction between the adjacent patterns cannot be neglected. For this reason, if several patterns having the same size are present on a reticle, the exposed and transferred images (i.e., photoresist images) on the wafer are thickened or thinned by influences of adjacent patterns of the reticle patterns having the same size.
According to an experimental conclusion, when the SHRINC method (particularly the filter SF.sub.2) is used, the resist line width of an isolated pattern and an end portion of a periodic pattern in the periodic direction tend to be smaller than that of the periodic pattern. The above conclusion is based on an assumption that patterns to be compared with each other have the same size on the reticle and are exposed and transferred with the same exposure amount, as a matter of course. In addition, if the isolated pattern is a linear pattern, the length of the line tends to slightly decrease.
In the SHRINC method, in order to obtain desired line widths of both the periodic pattern portion and the isolated pattern portion as resist images (i.e., the circuit pattern sizes obtained upon pattern etching), the shapes of the respective patterns of the reticle patterns must be corrected beforehand.
In the conventional SHRINC method, an algorithm (correction method) for automatically performing such correction and a correction apparatus are not realized at all.